KR102493977B1 - High-strength hot-dip galvanized steel sheet having good plating quality, steel sheet for hot-dip galvanizing and method of manufacturing thereof - Google Patents

Abstract

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent coating quality, a steel sheet for plating for manufacturing the same, and a manufacturing method thereof.
In the steel sheet for plating according to one aspect of the present invention, the GDS profile of the Mn component observed in the depth direction from the surface sequentially includes a maximum point and a minimum point, and the value obtained by dividing the Mn concentration at the maximum point by the Mn concentration of the base material and the minimum The difference between the value obtained by dividing the Mn concentration at the point by the Mn concentration in the base material (concentration difference in terms of Mn) is 80% or more, and the value obtained by dividing the Si concentration at the maximum point by the Si concentration in the base material and the Si concentration at the minimum point The difference between the values divided by the concentration of Si in the base material (the difference in concentration in terms of Si) may be 50% or more.
However, if a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is the point where the minimum point appears.

Description

High-strength hot-dip galvanized steel sheet with excellent plating quality, steel sheet for plating and their manufacturing method

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent coating quality, a steel sheet for plating for manufacturing the same, and a manufacturing method thereof.

In recent years, in the automobile industry, by applying high-strength steel sheets as steel materials for automobiles, safety has been improved and weight reduction due to thickness reduction has been achieved. Precipitation hardened steel, solid solution hardened steel, etc. have been developed as steel materials that can be preferably applied as steel for automobiles, and DP steel (Dual Phase Steel) and CP steel (Complex Phase Steel) using phase transformation to improve strength and elongation at the same time. steel), TRIP steel (Transformation Induced Plasticity Steel) and TWIP steel (Twinning Induced Plasticity Steel) were developed. These high-strength steels contain a variety of alloying elements compared to general steel. In particular, a lot of elements with a higher oxidation tendency than Fe, such as Mn, Si, Al, Cr, and B, are added.

In hot-dip galvanizing, the coating quality is determined by the surface condition of the annealed steel sheet immediately before plating. Plating properties deteriorate due to oxide formation. That is, during the annealing process, the elements diffuse to the surface and react with a small amount of oxygen or water vapor present in the annealing furnace to form single or complex oxides of the elements on the surface of the steel sheet, thereby reducing the reactivity of the surface. The surface of the annealed steel sheet with low reactivity interferes with the wettability of the hot-dip galvanizing bath, causing non-plating in which the plating metal is not attached locally or entirely to the surface of the coated steel sheet. The plating quality of the plated steel sheet is greatly deteriorated, such as peeling of the plated layer due to insufficient formation of the alloying suppression layer (Fe ₂ Al ₅ ).

Various technologies have been proposed to improve the plating quality of high-strength hot-dip galvanized steel sheets. Among them, Patent Document 1 controls the air-fuel ratio of air and fuel to 0.80 to 0.95 in the annealing process, oxidizes the steel sheet in a direct flame furnace in an oxidizing atmosphere, and Si, Mn, or Al to a certain depth inside the steel sheet A technique for providing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent plating quality is proposed by forming iron oxides including single or composite oxides, then performing reduction annealing of the iron oxides in a reducing atmosphere, and then performing hot-dip galvanizing. .

When using the method of reduction after oxidation in the annealing process as in Patent Document 1, components having a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized at a certain depth from the surface layer of the steel sheet and their diffusion to the surface layer is suppressed. Si, Mn, or Al alone or composite oxides are reduced, so wettability with zinc is improved, and non-plating can be reduced. However, in the case of steel with added Si, during the reduction process, Si is concentrated directly under the iron oxide to form a band-shaped Si oxide, which causes peeling in the surface layer including the plating layer, that is, between the reduced iron and the base iron below it. There is a problem in that it is difficult to secure the adhesion of the plating layer because peeling occurs at the interface.

On the other hand, as another method for improving the plating properties of high-strength hot-dip galvanized steel sheets, Patent Document 2 maintains a high dew point in the annealing furnace to internally oxidize easily oxidizable alloy components such as Mn, Si, and Al into the steel. A method of improving plating properties by reducing externally oxidized oxides on the surface of the steel sheet after annealing has been proposed. However, the method according to Patent Document 2 can solve the plating problem due to external oxidation of Si, which is easily internally oxidized, but the effect is insignificant when a large amount of Mn, which is relatively difficult to internally oxidize, is added. .

In addition, even if the plating property is improved by internal oxidation, linear non-plating occurs due to surface oxides formed unevenly on the surface, or when a hot-dip galvanized steel sheet (GA steel sheet) is manufactured through alloying heat treatment after plating Problems such as occurrence of linear defects due to non-uniform alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

As another prior art, there is a method of suppressing diffusion of alloying elements to the surface during annealing by performing pre-plating of Ni before annealing. However, this method is also effective in suppressing the diffusion of Mn, but has a problem in that it does not sufficiently suppress the diffusion of Si.

Korean Patent Publication No. 2010-0030627 Korean Patent Publication No. 2009-0006881

According to one aspect of the present invention, there is provided a hot-dip galvanized steel sheet excellent in plating quality in which unplating does not occur and the plating layer is peeled off, and a manufacturing method thereof.

According to another aspect of the present invention, there is provided a hot-dip galvanized steel sheet that can be manufactured into a hot-dip galvanized steel sheet having excellent surface quality without linear defects even when alloying heat treatment is performed after plating, and a manufacturing method thereof.

According to another aspect of the present invention, a steel sheet for plating capable of producing a hot-dip galvanized steel sheet having excellent coating quality and a manufacturing method thereof are provided.

The object of the present invention is not limited to the above. Those skilled in the art to which the present invention pertains will have no difficulty in understanding additional tasks of the present invention from the overall details of the present invention specification.

In the steel sheet for plating according to one aspect of the present invention, the GDS profile of the Mn component observed in the depth direction from the surface sequentially includes a maximum point and a minimum point, and the value obtained by dividing the Mn concentration at the maximum point by the Mn concentration of the base material and the minimum The difference between the value obtained by dividing the Mn concentration at the point by the Mn concentration in the base material (concentration difference in terms of Mn) is 80% or more, and the value obtained by dividing the Si concentration at the maximum point by the Si concentration in the base material and the Si concentration at the minimum point The difference between the values divided by the concentration of Si in the base material (the difference in concentration in terms of Si) may be 50% or more.

However, if a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is the point where the minimum point appears.

A hot-dip galvanized steel sheet, which is another aspect of the present invention, may include the above-described steel sheet for plating and a hot-dip galvanized layer formed on the steel sheet for plating.

Another aspect of the present invention, a method for manufacturing a steel sheet for plating, includes preparing a base iron; Forming an Fe plating layer containing 5 to 50% by weight of oxygen by performing electroplating on the base iron; And annealing by holding the base iron on which the Fe plating layer is formed at 600 to 950 ℃ for 5 to 120 seconds in an annealing furnace with 1 to 70% H ₂ -remaining N ₂ gas atmosphere controlled at a dew point temperature of -15 to +30 ℃ steps may be included.

Another aspect of the present invention, a method for manufacturing a hot-dip galvanized steel sheet, includes preparing a base iron; Forming an Fe plating layer containing 5 to 50% by weight of oxygen by performing electroplating on the base iron; The base iron on which the Fe plating layer is formed is annealed and plated by holding it at 600 to 950 ℃ for 5 to 120 seconds in an annealing furnace with a 1 to 70% H ₂ -residual N ₂ gas atmosphere controlled at a dew point temperature of -15 to +30 ℃ obtaining a steel sheet for use; and immersing the steel sheet for plating in a galvanizing bath containing Al: 0.1 to 0.3%, the rest of Zn, and unavoidable impurities, maintained at a temperature range of 440 to 500°C.

As described above, the present invention provides a hot-dip galvanized steel sheet that significantly improves the phenomenon of non-plating during hot-dip galvanizing and improves plating adhesion by forming a pre-plating layer and controlling the concentration profile of Mn and Si components inside. can provide

In addition, according to one aspect of the present invention, even if the hot-dip galvanized steel sheet of the present invention is subjected to alloying heat treatment, it is possible to prevent linear defects on the surface of the hot-dip galvanized steel sheet obtained, thereby providing an alloyed hot-dip galvanized steel sheet having excellent surface quality. can provide

1 is a schematic diagram of a GDS profile measured after removing a plating layer of a hot-dip galvanized steel sheet made of Fe electroplated cold-rolled steel sheet.
Figure 2 is a cross-sectional electron microscope photograph of a steel sheet annealed for 53 seconds at a dew point of +5 ℃, temperature 800 ℃, (a) is a cross-section of a steel sheet obtained by annealing without forming a Fe plating layer, (b) is It shows the cross section ^of the steel sheet obtained by electroplating Fe and annealing so that iron adhesion amount is 1.99g/m2.
3 is a schematic diagram of a process of annealing the base iron having an oxygen-containing Fe plating layer formed thereon in an atmosphere with a high dew point.
Figure 4 is a GDS concentration profile measured after removing the plating layer of a hot-dip galvanized steel sheet, (a) relates to the base iron of Comparative Example 2, (b) relates to the base iron of Comparative Example 11, (c) It relates to the iron base of Comparative Example 16, and (d) relates to the iron base of Inventive Example 7.

Hereinafter, a high-strength hot-dip galvanized steel sheet having excellent plating quality according to an aspect of the present invention completed through research by the present inventor will be described in detail. It should be noted that when the concentration of each element is expressed in the present invention, it means weight% unless otherwise specified. In addition, the Fe electroplating amount is a coating amount measured by the total amount of Fe included in the plating layer per unit area, and oxygen and unavoidable impurities in the plating layer are not included in the plating amount.

In addition, unless otherwise defined, concentration and concentration profile referred to in the present invention means concentration and concentration profile measured using GDS, that is, a glow discharge optical emission spectrometer.

Hereinafter, the present invention will be described in detail.

The cause of deterioration of unplating and plating adhesion in steel sheets containing a large amount of Mn and Si is attributed to surface oxides generated by oxidation of alloy elements such as Mn and Si on the surface during annealing of cold-rolled steel sheets at high temperatures. It is known.

It is a method of forming an oxide layer containing a large amount of oxygen in order to suppress the diffusion of alloy elements such as Mn and Si to the surface. It is a method of reducing oxidation by maintaining it in a reducing atmosphere after being oxidized during temperature rise, or a method of reducing iron oxide on the surface of a base metal. A method of coating and heat-treating may be used. However, the iron oxide firmly formed on the surface of the base metal is a mixture of not only FeO but also Fe ₃ O ₄ and Fe ₂ O ₃ , which are difficult to reduce. Since the base iron interface has a slow reduction rate, it is difficult to completely reduce it, and since Mn and Si oxides accumulate at the interface to form a continuous oxide layer, although wettability with molten zinc is improved, the oxide layer is easily crumbled and the plating layer This peeling problem may occur.

On the other hand, in the case of applying an annealing internal oxidation method in which alloy elements such as Mn and Si are oxidized inside the steel by increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process, Mn and Si oxides are preferentially formed on the steel surface during the heat treatment process. Then, Mn and Si are oxidized by oxygen diffused into the steel, suppressing surface diffusion. Therefore, a thin oxide film is formed on the surface of the base iron, but if the surface of the cold-rolled steel sheet before annealing is not completely homogeneous, or if local deviations such as oxygen partial pressure and temperature occur, non-plating occurs due to non-uniform wettability during hot dip galvanizing. In the process of alloying heat treatment after galvanizing, if the thickness of the oxide film is non-uniform and the difference in alloying degree occurs, it tends to cause linear defects that can be easily identified with the naked eye.

In order to solve the problems of the above technique, the present inventors tried to manufacture a hot-dip galvanized steel sheet with a beautiful surface and no plating peeling problem by controlling the presence of Mn and Si, which are oxidizing elements, on the surface of the steel sheet for plating as follows. .

That is, the steel sheet according to one embodiment of the present invention may have the following characteristics in the GDS concentration profile of Mn and Si. The steel sheet for plating of the present invention will be described in detail with reference to the GDS profile of FIG. 1 .

1 is a graph schematically showing a typical GDS profile of Mn components that may appear from a surface portion after removing a galvanized layer from a hot-dip galvanized steel sheet including the steel sheet of the present invention. In the graph, the vertical axis represents the concentration of alloy elements such as Mn and Si, and the horizontal axis represents the depth. As illustrated in the graph of FIG. 1, in the steel sheet for plating of the present invention, when the concentration profile of the Mn or Si component is directed from the surface (the interface with the plating layer in the case of hot-dip galvanization) to the inside, a maximum point and a minimum point appear sequentially. can have a shape. Here, having it sequentially does not necessarily mean that the maximum point appears first in the depth direction from the surface (interface). However, in some embodiments, the minimum point may not appear, and in this case, the internal concentration of the 5 μm depth region may be the minimum point concentration. In addition, the alloying element concentration of the surface has a value lower than the concentration of the maximum point, but in some cases, a minimum point having a low alloying element concentration may appear between the surface and the maximum point.

In the GDS concentration profile illustrated in FIG. 1, although not necessarily limited thereto, the surface layer portion corresponds to the Fe plating layer in which the concentration of the alloying element is low because the alloying element is not diffused much from the base iron, and the maximum point is between the Fe plating layer and the base iron. It corresponds to the region where internal oxides of alloy elements formed near the interface are concentrated, and the minimum point appearing on the base iron side of the Fe plating layer is the maximum point where alloy elements are diffused into the Fe plating layer that does not contain alloy elements and diluted or internal oxidation occurs. This corresponds to the region where alloying elements are diffused and depleted.

In one embodiment of the present invention, the maximum point may be formed at a depth of 0.05 to 1.0 μm from the surface of the steel sheet. If a maximum point appears in a region deeper than this, it may not be determined to be a maximum point due to the effect of the present invention. Also, the minimum point may be formed at a position within a depth of 5 μm from the surface of the steel sheet. As described above, if the minimum point is not formed at a point within a depth of 5 μm, the 5 μm depth may be used as the point where the minimum point is formed. Since the concentration at a depth of 5 μm is substantially equal to the concentration inside the base material, it can be regarded as a point at which the concentration no longer decreases.

At this time, in the Mn concentration profile and the Si concentration profile, the greater the difference between the converted concentration of the element at the maximum point (the value obtained by dividing the concentration at the corresponding point by the concentration of the base material, expressed in %) and the reduced concentration at the minimum point, the more Mn diffuses to the surface. and Si. In one embodiment of the present invention, in the case of Mn, the maximum point conversion concentration value - the minimum point conversion concentration value may be 80% or more, and the difference between the values in the case of Si may be 50% or more. Si is an element more oxidizable than Mn, and since it easily causes internal oxidation even inside the base iron having a low oxygen concentration, oxidation may occur in a wider area than Mn. Therefore, even if the difference between the maximum and minimum concentrations of Si is smaller than that of Mn, the degree of internal oxidation cannot be said to be small. As a result of experiments conducted by the present inventors under various conditions, it was possible to obtain a hot-dip galvanized steel sheet having good coating adhesion without occurrence of non-plating during hot-dip galvanizing when the above conditions are satisfied. However, if the difference between the maximum and minimum concentrations of Mn is less than 80%, or the difference between the maximum and minimum concentrations of Si is less than 50%, there is a problem of point or linear non-plating or plating peeling. There may be. That is, by doing this, it is possible to prevent the formation of oxides of Mn and Si on the surface, so that an ultra-high-strength hot-dip galvanized steel sheet with a beautiful surface and good plating adhesion can be manufactured. The occurrence of defects such as linear defects can be suppressed. Since the difference between the converted concentration values is more advantageous, there is no need to set an upper limit on the value. However, when considering the content of the elements included, the difference in the converted concentration value may be set to 400% or less in the case of Mn and 250% or less in the case of Si. In another embodiment of the present invention, the Mn conversion concentration difference may be 90% or more or 100% or more, and the Si conversion concentration difference may be 60% or more or 70% or more.

Hereinafter, the GDS analysis method performed in the present invention will be described in detail.

For GDS concentration analysis, the hot-dip galvanized steel sheet is sheared to a size of 30 to 50 mm in length, and immersed in a 5 to 10% by weight aqueous hydrochloric acid solution at room temperature of 20 to 25 ° C to remove the galvanized layer. In order to prevent damage to the surface of the base iron during the process of dissolving the galvanized layer, when the generation of bubbles due to the reaction between the galvanized layer and the acid solution stopped, the acid solution was removed within 10 seconds, and the base iron was washed with pure water and dried. Of course, if it is a steel sheet for plating that has not yet been hot-dip galvanized, it can be analyzed without such a coating layer removal operation.

The GDS concentration profile measures the concentrations of all components contained in the steel sheet every 1 to 5 nm in the thickness direction of the steel sheet. The measured GDS profile may contain irregular noise, and a Gaussian filter with a cutoff value of 100 nm was applied to the measured concentration profile to calculate the maximum and minimum points of the Mn and Si concentrations to obtain an average concentration profile, and the noise was removed. The concentration value and depth of the maximum and minimum concentration points were obtained from the profile, respectively. In addition, it should be noted that the maximum point and minimum point mentioned in the present invention were calculated as the maximum point and minimum point only when there was a difference of 10 nm or more from each other in the depth direction.

A steel sheet for plating that is targeted in the present invention may include a base iron and an Fe plating layer formed on the base iron. The composition of the iron base is not particularly limited.

However, in the case of a high-strength steel sheet containing 1.0 to 8.0% by weight of Mn and 0.3 to 3.0% by weight of Si to easily generate oxides on the surface, the present invention can advantageously improve plating properties. The upper limit of the concentration of Mn in the base iron is not particularly limited, but considering the commonly used composition, the upper limit may be limited to 8% by weight. In addition, the lower limit of the concentration of Mn is not particularly limited, but the composition containing less than 1.0% by weight of Mn has a beautiful surface quality of the hot-dip galvanized steel sheet even if the Fe plating layer is not formed, so Fe electroplating is not required. The upper limit of the Si concentration is not particularly limited, but considering the commonly used composition, the upper limit can be limited to 3.0% by weight or less, and when the Si concentration is less than 0.3% by weight, even if Fe electroplating and annealing internal oxidation are not performed simultaneously Since the hot-dip galvanizing quality is beautiful, there is no need to carry out the method of the present invention.

Since Mn and Si are elements that affect plating properties, their concentrations may be limited as described above, but the present invention does not particularly limit the other components of the base iron.

However, in consideration of the fact that non-plating and plating adhesion may be severely deteriorated in the case of a high-strength steel sheet containing a large amount of alloy components, in one embodiment of the present invention, the composition of the base iron in weight%, Mn: 1.0 ~ 8.0 %, Si: 0.3 to 3.0% C: 0.05 to 0.3%, Al: 0.005 to 3.0%, P: 0.04% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the remainder being Fe and unavoidable impurities. Here, the term "high strength" is used to include all cases in which high strength can be obtained by heat treatment in a subsequent process as well as cases in which high strength is obtained after annealing. In addition, in the present invention, high strength may mean 490 MPa or more based on tensile strength, but is not limited thereto. In addition to the above-mentioned components, the base iron may further include elements such as Ti, Mo, and Nb in a total amount of 1.0% or less. The base iron is not particularly limited, but in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet may be used as the base iron.

In one aspect of the present invention, a hot-dip galvanized steel sheet including the steel sheet for plating may be provided, and the hot-dip galvanized steel sheet may include a steel sheet for plating and a hot-dip galvanized layer formed on a surface of the steel sheet for plating. At this time, as the hot-dip galvanized steel sheet, any commercially available one can be applied, and the type is not particularly limited.

Next, one exemplary embodiment of a method for manufacturing a steel sheet for plating and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one embodiment of the present invention, the steel sheet for plating comprises the steps of preparing a base iron; Forming an Fe plating layer containing 5 to 50% by weight of oxygen by performing electroplating on the base iron; It can be manufactured by a process including the step of obtaining a steel sheet for plating by annealing the base iron on which the Fe plating layer is formed.

In FIG. 2, a 1.2 GPa class cold-rolled steel sheet containing 2.6% Mn, 1.0% Si, and other alloying elements was annealed for 53 seconds at a temperature of 800° C. in a N ₂ -5% H ₂ , dew point +5° C. atmosphere, and then cooled. Cross sections observed under a microscope are shown. During the entire heating time, the atmosphere was maintained the same, and during cooling, the dew point temperature was maintained at -40°C so that Fe was not oxidized. In FIG. 2, (a) is a cross-section of a steel sheet annealed without Fe plating, and (b) is a steel sheet annealed after electroplating so that the amount of Fe deposited on the cold-rolled steel sheet (base iron) is 1.99 g/m ² . is a cross section The oxygen content of the Fe plating layer was 6.3% by weight.

As shown in (a) of FIG. 2, it can be seen that fine Mn and Si oxides are observed from the surface layer of the steel sheet annealed at the dew point +5 ° C without Fe plating, and thick grain boundary oxides are formed inside the base iron. This is because grain boundary oxides begin to form from the stage in which the cold-rolled tissue recovers and recrystallizes into fine crystal grains in the process of temperature rise, and as the annealing temperature rises and the annealing time increases, oxygen flows into the inside of the steel whose grains are coarsened, leading to grain boundaries. This is because oxides of In this form, the concentrations of Mn and Si components gradually change in the GDS profile, so that the maximum and minimum points do not appear properly, or even if they appear, the difference in converted concentration does not satisfy the range limited by the present invention.

As shown in (b) of FIG. 2, when the Fe plating layer containing 5 to 50% by weight of oxygen is plated to an iron adhesion of 1.99 g / m ² and then annealed, oxide is hardly generated in the Fe plating layer region, Oxides in the form of particles are generated at the interface between the Fe plating layer and the base iron and inside the base iron, and these oxides act as nuclei of internal oxides, and linear oxides grow in a direction perpendicular to the surface of the steel sheet. However, the inner oxide formation depth is deeper when the Fe plating layer is not formed than when the Fe plating layer is formed. In this case, the Fe plating layer (surface layer) contains a small amount of Mn and Si components, and may have a depletion layer in which the Mn and Si contents in a region deeper than the maximum value are greatly reduced as well as showing maximum values at the interface.

On the other hand, when annealing is performed in a high dew point atmosphere without Fe plating, oxide is generated at the grain boundary of the microrecrystallized structure from the surface of the base iron to suppress crystal growth, resulting in irregular microcrystalline grains surrounded by microoxides. After forming a high plating layer, when annealed at a high dew point of -15℃ to +30℃, the Fe plating layer does not contain oxidizing alloy elements such as Mn and Si, so no oxide is generated at the grain boundary of the plating layer, and the Fe plating layer and the substrate Since oxide is generated at the iron interface, the structure of the Fe plating layer having a uniform thickness and the crystal grains inside the base iron are distinguished. However, depending on the dew point in the annealing furnace, the elongation rate of the base iron, and the steel composition, there are cases where the boundary between the Fe plating layer and the base iron does not appear clearly. Even if it is controlled, it does not necessarily have the same characteristics as in FIG. 2(b).

Unlike the oxidation-reduction method, the internal oxidation method of annealing does not form a layered oxide layer, so it exhibits excellent characteristics in improving plating adhesion during hot-dip galvanization of ultra-high strength steel sheets containing a large amount of alloying elements such as Mn and Si. Since water vapor inevitably first oxidizes the surface of the steel sheet and then oxygen penetrates into the inside, the surface oxide cannot be fundamentally removed.

In order to solve the above problems, the inventors of the present inventors form an Fe plating layer containing a large amount of oxygen through many experiments and then annealing in a high dew point atmosphere, oxygen from water vapor in the annealing furnace on the surface of the Fe plating layer It was found that the diffusion of oxygen contained in the Fe plating layer to the surface by internal oxidation of alloy elements such as Mn and Si in the base iron can be effectively suppressed even without the formation of surface oxides of elements. Oxygen introduced into the steel due to the high dew point in the annealing furnace additionally oxidizes alloying elements, so surface oxides of alloying elements are hardly generated on the steel surface, which dramatically improves the surface quality and coating adhesion of hot-dip galvanized steel sheets. , Even when manufacturing an alloyed hot-dip galvanized steel sheet, it is possible to obtain a uniform hot-dip galvanized steel sheet without surface defects by accelerating the alloying reaction.

More specifically, an Fe plating layer containing 5 to 50% by weight of oxygen is formed on a cold-rolled steel sheet (base iron), and the mechanical properties of the steel sheet can be secured in an annealing furnace controlled at a dew point of -15 ℃ to +30 ℃ When the temperature is raised to a temperature of 600 to 950 ° C. and then cooled again to perform hot-dip plating, non-plating is suppressed and a hot-dip galvanized steel sheet having excellent coating adhesion can be obtained.

In one embodiment of the present invention, the Fe plating layer may be formed through a continuous plating process, and at this time, the amount of Fe coating may be 0.5 to 3.0 g/m ² based on the amount of Fe deposited. When the Fe plating amount is less than 0.5 g/m ² , the effect of suppressing the diffusion of alloying elements by the Fe plating layer may be insufficient in a typical continuous annealing process. In addition, even if it exceeds 3.0g/m ² , the inhibitory effect of the alloying element can be further increased, but a plurality of plating cells must be operated to secure a high coating amount, and the electroplating solution is rapidly acidified when an insoluble anode is used. As a result, the plating efficiency is lowered, and sludge is generated, which is not economical. In another embodiment of the present invention, the Fe plating amount may be 1.0 to 2.0 g/m ² . When internal oxidation is performed after forming the Fe plating layer, internal oxides are formed at the interface or directly below the interface between the Fe plating layer and the base iron, so the maximum concentration of Mn and Si exists in the range of 0.05 to 1.0 μm. The Fe coating weight of 0.5 to 3.0 g/m ² of the present invention may correspond to a thickness of 0.05 to 0.4 μm after annealing.

In addition, the Fe plating layer having the above-described high oxygen concentration is formed in the maximum point and minimum point in the GDS concentration profile of Mn and Si elements inside the steel sheet for plating by controlling the temperature, dew point temperature and atmosphere of the subsequent annealing process, and at the maximum point The reduced concentration and the reduced concentration at the local minimum can satisfy the numerical range limited in one embodiment of the present invention. Considering this point, in one embodiment of the present invention, the oxygen concentration in the Fe plating layer may be 5 to 50% by weight, and in another embodiment, it may be 10 to 40% by weight. In order to obtain the surface oxide suppression effect, the amount of oxygen in the Fe plating layer must be sufficiently large. Even if the oxygen concentration in the Fe plating layer is less than ⁵ % by weight, it is possible to obtain the effect of suppressing the surface oxide by increasing the amount of Fe coating. can In addition, when the oxygen content is less than 5% by weight, it is difficult to sequentially form maximum points and minimum points in the GDS profile of Mn and Si. control over On the other hand, as the oxygen concentration in the Fe plating layer may increase, the effect of suppressing surface oxide during annealing may further increase. can do. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer may be limited to 10 to 40%.

In one embodiment of the present invention, the annealing temperature may be 600 ° C to 950 ° C based on the steel sheet temperature of the soak zone. If the annealing temperature is too low, the cold-rolled steel sheet structure is not properly recovered and recrystallized, making it difficult to secure mechanical properties such as strength and elongation of the steel sheet. Plating quality is poor, and it is not economical because it is operated at a high temperature unnecessarily.

Meanwhile, in one embodiment of the present invention, the dew point inside the annealing furnace may be -15°C to +30°C. When the dew point is less than -15 ° C, the amount of oxygen flowing into the steel decreases and only the surface oxidation is increased and internal oxidation does not occur, so that a large amount of oxide is present on the surface, resulting in poor hot-dip galvanizing quality. In addition, even if the dew point exceeds +30 ℃, internal oxidation increases, suppressing the diffusion of alloying elements, thereby suppressing surface oxidation, but the effect of suppressing surface oxidation increases. Equipment problems may occur if the steam is condensed and applied for a long time in continuous annealing. The dew point can be managed in the above-described range at 600 to 950 ° C., and can be managed under more relaxed conditions in a lower temperature range. In another embodiment of the present invention, the dew point may be limited to -10 to +20 °C.

In addition, in order to prevent oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmosphere gas during annealing may be set to 1% or more in terms of volume%. When the hydrogen concentration is less than 1%, a small amount of oxygen inevitably included in the H ₂ and N ₂ gas cannot be effectively oxidized and removed, and the oxygen partial pressure increases, which may cause surface oxidation of the ferrous metal. On the other hand, if the hydrogen concentration exceeds 70%, the risk of explosion in case of gas leakage and the cost of high hydrogen work increase, so the hydrogen concentration can be set to 70% or less. It may be substantially nitrogen (N ₂ ) except for impurity gases that are inevitably included other than hydrogen (H ₂ ).

And, according to one embodiment of the present invention, the holding time after reaching the target temperature during annealing may be limited to 5 to 120 seconds. During annealing, it is necessary to maintain the annealing target temperature for 5 seconds or more in order to sufficiently transfer heat to the inside of the base steel to obtain uniform mechanical properties in the thickness direction. On the other hand, if the high-temperature annealing holding time is excessively long, the diffusion of alloying interfering elements through the Fe plating layer increases, resulting in an increase in the amount of surface oxides produced, and as a result, the hot-dip galvanizing quality deteriorates, so it can be limited to 120 seconds or less.

Hereinafter, the effect of suppressing surface diffusion of Mn and Si during annealing of a cold-rolled steel sheet having a Fe plating layer containing a large amount of oxygen based on the above description in a high dew point atmosphere will be described in detail with reference to FIG. 3 .

3 schematically illustrates phenomena occurring inside the steel sheet as the temperature of the steel sheet is raised according to the conditions of the present invention. 3(a) shows a schematic cross-sectional view of a base iron having an Fe plating layer containing a large amount of oxygen. The base iron contains alloy elements such as Mn and Si, and the Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed during electroplating, and the balance is composed of Fe.

3(b) shows a state in which the Fe-plated cold-rolled steel sheet is heated to about 300 to 500° C. in a nitrogen atmosphere containing 1 to 70% H ₂ . The surface of the Fe plating layer containing a large amount of oxygen is gradually reduced and oxygen is removed, but at the interface between the Fe plating layer and the base iron, Mn and Si diffused from the base iron combine with oxygen in the Fe plating layer to form internal oxides. diffusion is inhibited. In addition, as the temperature increases, as Mn and Si diffused inside the base iron accumulate, the internal oxide at the interface gradually grows. Even if the dew point in the annealing furnace varies widely from -90 ° C to + 30 ° C in the low-temperature region of the temperature rise step, a large amount of oxygen exists in the Fe plating layer than the rate at which oxygen dissociated from water vapor diffuses into the steel due to the low temperature, Fe Since the plating layer is reduced and the rate at which oxygen is released is faster, even if the dew point in the annealing furnace changes in the low-temperature section, it is not greatly affected. Therefore, dew point control is not a very important factor in this step.

However, the amount of oxygen inside the Fe plating layer plays an important role, and if a lot of fine internal oxides are generated at the interface between the Fe plating layer and the base iron and inside the base iron at the low temperature stage, the alloy elements inside the base iron will continue to be internally oxidized. It can act as an oxide nucleus. In order to generate such oxide nuclei, the concentrations of oxygen and alloy components must be high at the same time. produced in large quantities However, if almost no oxygen is included in the Fe plating layer, alloying elements included in the base iron pass through the Fe plating layer to form oxides on the surface. When the temperature is raised thereafter, oxygen in the Fe plating layer is further depleted, and the diffusion of alloying elements in the base iron is further increased, so that surface oxide formation increases.

Figure 3 (c) shows a cross-sectional schematic of the base iron when the temperature is raised to 500 ~ 700 ℃ in the same reducing atmosphere. During the temperature raising process, it is preferable to control the dew point of the inside of the annealing furnace to -15°C to +30°C. When the temperature rises, the Fe plating layer is sufficiently reduced and the oxygen concentration is lowered, so the oxygen release rate slows down, while the water vapor in the annealing furnace dissociates and the diffusion rate into the steel greatly increases. Therefore, when the dew point is raised from the range of 500 to 700 ° C., which is lower than the temperature at which the Fe plating layer is completely reduced, diffusion of Mn and Si inside the steel to the surface through the Fe plating layer can be effectively suppressed.

Figure 3 (d) shows a cross-sectional schematic of the steel sheet after maintaining at a high temperature in the range of 600 ~ 950 ℃ while adjusting the dew point to -15 ℃ to +30 ℃. Inside the base iron, Mn and Si are continuously diffused, and on the surface of the steel sheet, oxygen supplied from water vapor quickly penetrates and is supplied, so Mn and Si are oxidized inside. Mn and Si oxides in the form of particles generated by reacting with oxygen act as nuclei in which oxides can grow, so the inner oxides grow concentrated on the interface between the Fe plating layer and the base iron. In addition, since the diffusion rate of oxygen is faster than that of Mn and Si, which have a large atomic size, internal oxides are formed deeply not only at grain boundaries but also inside grains.

Although the control conditions for each temperature have been described above, the most important step in the annealing process is to maintain the steel sheet temperature at 600 to 950 ° C. In this temperature range, the distribution of oxides inside the steel sheet for plating can be controlled simply by controlling the dew point of the atmosphere. can be effectively controlled. Of course, this dew point control is not particularly problematic even if it is performed in all processes prior to the maintaining step. In addition, it should be noted that the above process is merely exemplifying one embodiment of the present invention in the description, and the reaction mechanism of the present invention is not always construed as being constrained to the above description.

After the annealing step, the annealed steel sheet may be cooled. Since the cooling conditions in the cooling step after the annealing step do not significantly affect the surface quality of the final product, that is, the plating quality, there is no need to specifically limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere may be applied to at least iron.

According to one embodiment of the present invention, a hot-dip galvanized layer may be formed by performing hot-dip galvanizing on the steel sheet for plating obtained by the above-described process. In the present invention, the hot-dip galvanizing method is not particularly limited.

In addition, in the present invention, since any base iron having the above-described alloy composition can be applied without limitation as a base iron for plating steel sheet or hot-dip galvanized steel sheet according to the present invention, the method for manufacturing base iron may not be specifically limited.

In one embodiment of the present invention, the Fe plating layer can be formed on the surface of the base iron through an electroplating method, and the oxygen concentration of the Fe plating layer formed can be controlled by appropriately controlling the conditions of the electroplating solution and the plating conditions.

That is, in order to form the Fe plating layer in the present invention, iron ions including ferrous ions and ferric ions; complexing agent; and unavoidable impurities, and the concentration of ferric ions among the iron ions may be 5 to 60% by weight of the electroplating solution.

According to one embodiment of the present invention, the electroplating solution contains ferrous ions and ferric ions. In order to obtain high plating efficiency, it may be advantageous to include only ferrous ions. However, if only ferrous ions are included, the solution deteriorates and the plating efficiency rapidly decreases, which may cause quality deviation in the continuous electroplating process. , The ferric ion may be further included. At this time, the concentration of the ferric ions is preferably 5 to 60% by weight, more preferably 5 to 40% by weight of the total amount of ferrous and ferric ions. If it is less than 5%, the rate at which ferric iron is reduced to ferrous iron at the cathode is smaller than the rate at which ferrous iron is oxidized to ferric at the anode, so that the concentration of ferric iron rises rapidly and the pH drops rapidly, resulting in a decrease in plating efficiency. continuously deteriorate On the other hand, when the concentration of ferric ions exceeds 60%, the amount of reaction in which ferric iron is reduced to ferrous iron at the cathode increases more than the amount of reaction in which ferrous iron is reduced and precipitated as metallic iron, so the plating efficiency is greatly reduced. and the plating quality deteriorates. Therefore, in consideration of equipment and process characteristics such as plating amount, working current density, solution replenishment amount, amount of solution lost on the strip, and concentration change rate due to evaporation, the concentration of ferric ions among the iron ions is 5 to 60% by weight It is desirable to have

The iron ion concentration is preferably 1 to 80 g per 1 L of the electroplating solution, and more preferably 10 to 50 g per 1 L. If it is less than 1 g/L, there is a problem in that plating efficiency and plating quality rapidly deteriorate. On the other hand, if it exceeds 80 g/L, precipitation may occur due to exceeding solubility, and raw material loss due to loss of solution in the continuous plating process may occur. increase, so it is not economical.

The electroplating solution of the present invention includes a complexing agent, and it is preferable to use an amino acid or an amino acid polymer as the complexing agent in order to maintain a high plating efficiency without generating sludge while containing a large amount of ferric iron.

An amino acid refers to an organic molecule in which a carboxyl group (-COOH) and an amine group (-NH ₂ ) are bonded, and an amino acid polymer refers to an organic molecule formed by polymerization of two or more amino acids, and an amino acid polymer is a complexing agent similar to an amino acid. represents a characteristic. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

When amino acids are dissolved in neutral water, amines combine with hydrogen ions to have positive charges, and carboxyl groups have negative charges when hydrogen ions dissociate, so amino acid molecules maintain charge neutrality. On the other hand, when the solution is acidified, the carboxyl group recombines with the hydrogen ion to become charge neutral, and since the amine has a positive charge, the amino acid molecule forms a cation. That is, amino acids form charge neutral or positive ions in weakly acidic aqueous solutions.

When amino acids are added to an acidic electrolyte containing iron ions, they are complexed with ferrous and ferric ions, and the iron ions complexed with amino acids maintain a positive ion state even in a complexed state. Therefore, a typical complexing agent having a plurality of carboxyl groups exhibits characteristics electrically opposite to those of being negatively charged in a weakly acidic aqueous solution.

In addition, compared to complexing agents containing a plurality of carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and have weak bonding strength, but their bonding strength with ferric ions that generate sludge is sufficiently strong. Precipitation by ions can be prevented. In addition, since ferric ions can maintain positive ions even when complexed, ferric ions are easily transferred to the negative electrode and reduced to ferrous ions to participate in the plating reaction, whereas movement to the positive electrode is inhibited, resulting in ferric iron Since the rate of generation of ions slows down, even if continuous plating is performed for a long period of time, the concentration of ferric ions is maintained at a constant level, the plating efficiency is maintained constant, and there is no need to replace the electrolyte.

On the other hand, in the continuous electroplating process, when the iron ions in the solution are consumed by plating, the solution is acidified. Even if the same amount of iron ions are precipitated, the solution containing ferric ions has a higher pH than the solution containing only ferrous ions. change will decrease. When the pH increases, some ferric ions combine with hydroxyl ions, and when the pH decreases, hydroxyl ions are separated and neutralized, so the solution containing ferric ions slows down the pH change even without a separate pH buffer, acting as a pH buffer. Therefore, it is possible to keep the electroplating efficiency constant in the continuous electroplating process.

Therefore, sludge can be prevented by using amino acids as a complexing agent, ferric ions as well as ferrous ions can be used as plating raw materials, and when ferrous ions and ferric ions are mixed and used, a solution Since the change in pH is slowed down and the accumulation of ferric ions can be easily prevented, the electroplating efficiency and plating quality can be kept constant in a continuous electroplating process.

On the other hand, the complexing agent is added in an amount such that the molar concentration ratio of the iron ion and the complexing agent is 1: 0.05 to 2.0, more preferably 1: 0.5 to 1.0. If it is less than 0.05, the excessive ferric ions combine with hydroxyl ions or oxygen to prevent sludge formation, and even if ferric iron is not included, the plating efficiency is greatly reduced, and furthermore, burning is caused, resulting in poor plating quality. it gets worse On the other hand, even if it exceeds 2.0, the sludge suppression effect and plating quality are maintained, but the overvoltage rises, resulting in a decrease in plating efficiency. It is not economical because the cost rises.

The complexing agent is preferably one or more selected from amino acids or amino acid polymers, and may be, for example, one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

When electroplating is performed at a current density of 3 to 120 A/dm ² while maintaining a solution temperature of 80° C. or less and a pH of 2.0 to 5.0 using the amino acid as a complexing agent, an Fe plating layer with high plating efficiency and high oxygen concentration can be obtained. can

The temperature of the Fe electroplating solution does not greatly affect the quality of the Fe plating layer, but when it exceeds 80 ° C., the evaporation of the solution becomes severe and the concentration of the solution continuously changes, making uniform electroplating difficult.

If the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency is lowered and is not suitable for the continuous plating process. If the pH exceeds 5.0, the plating efficiency increases, but during continuous electroplating, sludge in which iron hydroxide is precipitated is generated. This causes clogging of pipes, contamination of rolls and equipment.

When the current density is less than 3A/dm ² , the plating overvoltage of the cathode decreases and the Fe electroplating efficiency decreases, so it is not suitable for continuous plating process, and when the current density exceeds 120A/dm ² , burning occurs on the plating surface, resulting Problems arise that the plating layer is non-uniform and the Fe plating layer is easily removed.

As described above, the present invention preferably contains 5 to 50% by weight of oxygen in the Fe plating layer. The causes of mixing of oxygen in the Fe plating layer are as follows. In the process of depositing iron on the surface of the steel sheet to which the cathode is applied, hydrogen ions are reduced to hydrogen gas at the same time, and the pH rises. Therefore, both ferrous and ferric ions temporarily bond with OH ⁻ ions and can be incorporated together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the iron ion combined with the OH ^- ion of the complexing agent becomes negatively charged on average, and when a cathode is applied for electroplating, an electrical repulsive force generated and the mixing of Fe into the plating layer is suppressed. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and have positive ions in strong acids below pH 2.0. Even when 1 or 2 OH ^- are combined with iron ions bound to amino acids, they have positive ions, so it is a cathode for electroplating. Oxygen is mixed in a large amount due to the occurrence of electric attraction. Therefore, when Fe electroplating is performed by using amino acids as a complexing agent so that the molar concentration ratio of iron ions and amino acids is 1:0.05 to 1:2.0 and maintaining pH 2.0 to 5.0, plating efficiency is high and sludge generation is suppressed However, an Fe plating layer containing 5 to 50% by weight of oxygen can be obtained.

In order to secure the hot-dip galvanizing quality of the steel sheet containing Mn and Si, it is preferable to treat the coating amount of the Fe plating layer at 0.5 to 3.0 g/m ² based on the iron concentration. The upper limit of the Fe coating amount is not particularly limited, but if it exceeds 3.0 g/m ² in a continuous plating process, it is not economical because a plurality of plating cells are required or the production rate is lowered. In addition, when the Fe electroplating amount is large, the Fe electroplating solution is rapidly denatured in a continuous process, and the pH decreases and the plating efficiency is greatly reduced, making it difficult to manage the solution. On the other hand, if the Fe electroplating amount is less than 0.5 g/m ² , since oxygen contained in the Fe plating layer is rapidly reduced and removed, it is not possible to effectively suppress the formation of surface oxides due to diffusion of Mn and Si from the base iron, thereby preventing hot dip plating. There is a problem of deterioration in quality. The Fe plating amount is the concentration of iron contained in the plating layer and has a thickness of about 0.05 to 0.4 μm when the Fe plating layer is completely reduced during annealing.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for exemplifying and specifying the present invention, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

First, one type of iron was prepared as shown in Table 1 below. The base iron was a cold-rolled steel sheet, and no special plating layer was formed on the surface.

ingredient Mn Si C Al Ti P S B Mo Cr Fe density 2.68 1.46 0.17 0.046 0.024 0.021 0.005 0.0022 0.06 0.5 balance

Prior to performing Fe electroplating on the steel sheet, after Fe electroplating was performed using a Cu plate, the total amount of Fe was measured by dissolving in a 5 to 10% by weight hydrochloric acid solution to measure the amount of electroplating and plating efficiency in advance. Referring to the measured plating efficiency, by performing Fe electroplating on the cold-rolled steel sheet, the Fe electroplating amount was similarly adjusted even if the plating solution and plating conditions were changed. Fe electroplating was additionally performed on the Cu plate under each solution and plating condition, and the total amount of Fe and O was obtained through GDS analysis, and the average oxygen concentration of the Fe plating layer according to each electroplating condition was measured. was measured separately and is shown in Table 2. During plating, the temperature of all plating solutions was adjusted to 50°C.

On the other hand, after forming Fe plating layers on several cold-rolled steel sheets having the composition shown in Table 1 under the same conditions as the electroplating solution and plating conditions for Cu, annealing and hot-dip galvanization were performed under the following conditions.

Inside the annealing furnace, a reducing atmosphere was maintained with an N ₂ gas atmosphere containing 5% H ₂ in all sections, and the dew point was maintained at -18 ° C to +5 ° C only until the temperature elevation section and cracking zone section, as shown in Table 2, , It was maintained at -40 ° C from the cooling section. Here, the cold-rolled steel sheet electroplated with Fe was charged in the above-described process (i.e., conditions in Table 2, plating bath temperature: 50 ° C.), heated to 850 ° C. at a heating rate of about 2.5 ° C./sec, and then maintained for 53 seconds. After that, it was slowly cooled to 650 ° C at a rate of 2.8 ° C / sec and rapidly cooled to 400 ° C at a rate of 14.5 ° C. When the cooling was completed, the temperature was raised again to 480 ° C so that hot-dip galvanizing was possible, and the hot-dip galvanizing bath was introduced into a hot-dip galvanizing bath containing 0.20 to 0.25% of Al, and the temperature was maintained at 460 ° C. The hot-dip galvanized steel sheet was prepared by slowly cooling to room temperature.

Plating properties of the manufactured hot-dip galvanized steel sheets were evaluated, and the GDS concentration profile was measured for the base iron in which the plating layer was dissolved with about 8% hydrochloric acid solution, and the maximum and minimum points of Mn and Si and 5㎛ inside the base iron The average concentration was measured in each, and the results are shown in Table 3.

Coating properties of the hot-dip galvanized steel sheet were visually evaluated. If there is no non-plating over the entire area, it is indicated as 'good', and if fine dotted non-plating within 1 mm occurs, it is classified as 'dot unplating', and linear non-plating or dot-shaped non-plating is linear or clustered. Therefore, if several defects occurred at the same time, it was marked as 'linear defect', and if non-plating occurred in a large area of 5 mm or more in diameter, it was classified as 'non-plating'. Plating defects tend to have large unplated area ratios in the order of 'non-plating', 'linear defects', 'dot non-plating', and 'good'.

To evaluate coating adhesion, automotive structural sealer was applied to a thickness of about 5 mm on the hot-dip galvanized steel sheet and cured at a temperature of 150 to 170 ° C. The hot-dip galvanized steel sheet cooled to room temperature was bent at 90 degrees to remove the sealer. If the plating layer adheres to the sealer and peels off at the entire interface between zinc plating and base iron, the plating adhesion is judged to be poor and marked as 'peeling', and when the plating layer does not peel off, the plating adhesion is judged to be 'good' did In some specimens, only a part of the plating layer was peeled off, and in this case, it was marked as 'partial peeling'. However, the coating adhesion was not evaluated for the specimens in which 'unplating' occurred.

The concentration profile of the ferrous iron having the plating layer removed with hydrochloric acid was obtained according to the above-described GDS analysis method, and after removing noise by applying a 100 nm Gaussian filter, maximum points and minimum points were calculated. In some GDS profiles, maxima or minima could not be calculated, and in these cases, it was marked as 'ND'. However, even if it is not indicated in the case of the minimum point, when calculating the difference in conversion concentration, it was introduced into the calculation by considering it to be the same as the concentration of Mn and Si inside the base material described below. However, when the maximum point was not formed, the converted concentration could not be obtained, and it was considered to be out of the scope of the present invention.

As the concentrations of Mn and Si in the base material, the values measured at 5 μm in the depth direction from the surface of the steel sheet (interface between the galvanized layer and the steel sheet) were used.

division iron concentration
(g/L) Ferric iron concentration (g/L) Type of complexing agent Complexing agent/iron molarity ratio current density
(A/dm ² ) pH Fe electroplating amount
(g/m ² ) Oxygen concentration in the Fe plating layer
(weight%) Dew Point in Annealing Furnace
(℃) Comparative Example 1 0 -15℃ Comparative Example 2 0 +5℃ Comparative Example 3 50.1 4.2 citric acid 0.2 40 3.20 0.42 4.6 -15℃ Comparative Example 4 50.1 4.2 citric acid 0.2 40 3.20 0.81 3.3 -15℃ Comparative Example 5 50.1 4.2 citric acid 0.2 40 3.20 1.21 4.9 -15℃ Comparative Example 6 50.1 4.2 citric acid 0.2 40 3.20 1.99 4.3 -15℃ Comparative Example 7 50.1 4.2 citric acid 0.2 40 3.20 2.99 4.3 -15℃ Comparative Example 8 50.1 4.2 citric acid 0.2 40 3.20 0.42 4.6 +5℃ Comparative Example 9 50.1 4.2 citric acid 0.2 40 3.20 0.81 3.3 +5℃ Comparative Example 10 50.1 4.2 citric acid 0.2 40 3.20 1.21 4.9 +5℃ Comparative Example 11 50.1 4.2 citric acid 0.2 40 3.20 1.99 4.3 +5℃ Comparative Example 12 50.1 4.2 citric acid 0.2 40 3.20 2.99 4.3 +5℃ Comparative Example 13 49.2 4.8 glycine 0.5 20 3.00 0.40 10.3 -18℃ Comparative Example 14 49.2 4.8 glycine 0.5 20 3.00 0.82 8.7 -18℃ Comparative Example 15 49.2 4.8 glycine 0.5 20 3.00 1.18 9.2 -18℃ Comparative Example 16 49.2 4.8 glycine 0.5 20 3.00 1.99 6.3 -18℃ Comparative Example 17 49.2 4.8 glycine 0.5 20 3.00 3.00 5.1 -18℃ Comparative Example 18 49.2 4.8 glycine 0.5 20 3.00 0.40 10.3 -15℃ Invention example 1 49.2 4.8 glycine 0.5 20 3.00 0.82 8.7 -15℃ Invention example 2 49.2 4.8 glycine 0.5 20 3.00 1.18 9.2 -15℃ Inventive example 3 49.2 4.8 glycine 0.5 20 3.00 1.99 6.3 -15℃ Inventive example 4 49.2 4.8 glycine 0.5 20 3.00 3.00 5.1 -15℃ Comparative Example 19 49.2 4.8 glycine 0.5 20 3.00 0.40 10.3 +5℃ Inventive Example 5 49.2 4.8 glycine 0.5 20 3.00 0.82 8.7 +5℃ Inventive example 6 49.2 4.8 glycine 0.5 20 3.00 1.18 9.2 +5℃ Inventive Example 7 49.2 4.8 glycine 0.5 20 3.00 1.99 6.3 +5℃ Inventive Example 8 49.2 4.8 glycine 0.5 20 3.00 3.00 5.1 +5℃ Inventive Example 9 50.7 5.1 glycine 0.5 70 2.90 0.50 45.1 -15℃ Inventive Example 10 50.7 5.1 glycine 0.5 70 2.90 1.01 42.7 -15℃ Inventive Example 11 50.7 5.1 glycine 0.5 70 2.90 2.01 39.4 -15℃ Inventive example 12 50.7 5.1 glycine 0.5 70 2.90 2.98 37.2 -15℃ Inventive Example 13 50.7 5.1 glycine 0.5 70 2.90 0.50 45.1 +5℃ Inventive Example 14 50.7 5.1 glycine 0.5 70 2.90 1.01 42.7 +5℃ Inventive Example 15 50.7 5.1 glycine 0.5 70 2.90 2.01 39.4 +5℃ Inventive Example 16 50.7 5.1 glycine 0.5 70 2.90 2.98 37.2 +5℃

division Plating property Plating Adhesion Mn peak concentration
(weight%) Mn minimum concentration
(weight%) Mn concentration inside the parent material
(weight%) Si maximum concentration
(weight%) Si minimum concentration
(weight%) Si concentration inside the parent material
(weight%) Comparative Example 1 Unplated - ND ND 2.70 ND ND 1.46 Comparative Example 2 dot unplated Partial exfoliation 4.15 2.43 2.69 1.74 1.30 1.44 Comparative Example 3 Unplated - ND ND 2.67 ND ND 1.45 Comparative Example 4 Unplated - ND ND 2.67 ND ND 1.44 Comparative Example 5 Unplated - 3.55 1.67 2.66 2.07 ND 1.44 Comparative Example 6 linear fault exfoliation 3.18 1.26 2.70 1.67 1.13 1.47 Comparative Example 7 linear fault exfoliation 2.82 0.78 2.68 3.24 0.87 1.45 Comparative Example 8 linear fault exfoliation 3.96 2.26 2.69 1.57 1.13 1.48 Comparative Example 9 linear fault exfoliation 3.88 2.10 2.66 1.55 1.09 1.44 Comparative Example 10 dot unplated exfoliation 3.75 1.95 2.70 1.58 1.13 1.45 Comparative Example 11 Good exfoliation 3.86 1.96 2.69 1.86 1.17 1.48 Comparative Example 12 Good Partial exfoliation 3.91 1.89 2.69 1.93 1.18 1.46 Comparative Example 13 linear fault exfoliation ND ND 2.66 ND ND 1.44 Comparative Example 14 linear fault exfoliation ND ND 2.68 ND ND 1.46 Comparative Example 15 linear fault exfoliation ND ND 2.68 ND ND 1.48 Comparative Example 16 Good exfoliation 3.72 2.28 2.68 1.32 0.67 1.44 Comparative Example 17 Good Partial exfoliation 4.04 2.02 2.68 2.25 1.23 1.47 Comparative Example 18 linear fault Partial exfoliation 4.19 2.49 2.66 2.10 ND 1.44 Invention example 1 Good Good 4.31 2.04 2.68 2.69 1.43 1.46 Invention example 2 Good Good 4.64 1.64 2.68 3.13 1.32 1.48 Inventive example 3 Good Good 5.09 1.11 2.68 3.46 1.11 1.44 Inventive example 4 Good Good 4.77 0.74 2.68 3.22 0.84 1.47 Comparative Example 19 Good Partial exfoliation 4.21 2.26 2.67 1.68 1.03 1.47 Inventive Example 5 Good Good 5.29 1.77 2.70 2.00 1.15 1.47 Inventive example 6 Good Good 5.90 1.82 2.68 2.21 1.12 1.45 Inventive Example 7 Good Good 6.13 1.82 2.67 2.46 1.20 1.48 Inventive Example 8 Good Good 6.60 1.42 2.68 2.50 1.16 1.44 Inventive Example 9 Good Good 5.83 2.18 2.67 2.39 1.52 1.44 Inventive Example 10 Good Good 6.03 1.83 2.66 2.60 1.42 1.47 Inventive Example 11 Good Good 5.93 1.39 2.68 2.48 1.16 1.46 Inventive example 12 Good Good 6.28 0.92 2.69 2.41 0.94 1.45 Inventive Example 13 Good Good 5.66 1.91 2.68 2.16 1.16 1.46 Inventive Example 14 Good Good 6.18 1.91 2.66 2.47 1.24 1.47 Inventive Example 15 Good Good 6.60 1.99 2.69 2.59 1.24 1.46 Inventive example 16 Good Good 6.90 1.49 2.67 2.84 1.26 1.47

In Comparative Examples 1 and 2, annealing conditions and hot-dip galvanizing were performed on the base iron without the Fe plating layer in the same conditions as described above. In Comparative Example 1, when Fe electroplating is not performed and annealing is performed while maintaining the dew point in the annealing furnace at -15 ° C, hot-dip galvanization is not performed and only surface oxidation occurs, so the concentration of Mn and Si on the surface is high in the GDS profile. The concentration tended to gradually decrease in the inside of the base steel, and the maximum and minimum points could not be calculated. On the other hand, in Comparative Example 2, the dew point in the annealing furnace was maintained at +5° C., fine dotted non-plating occurred on the hot-dip galvanized surface, and peeling occurred partially during plating adhesion evaluation. The GDS concentration profile of the iron base of Comparative Example 2 is shown in the graph (a) of FIG. 4 . In the figure, the solid line is the concentration profile of Mn, and the dotted line is the concentration profile of Si (hereinafter the same). A large amount of internal oxide was generated within 0.5 μm of the inside of the ferrous metal rather than the surface, and had a maximum point, but the Mn depletion layer occurred at a depth of 2 μm from the surface. This is because oxygen continuously flows into the steel during annealing to form grain boundary internal oxidation deep to the inside of the steel, which can be seen in (a) of FIG. In Comparative Example 2, the difference between the converted concentrations at the maximum and minimum points of Mn and Si was 63.9% and 30.8%, respectively, showing insufficient results.

In Comparative Examples 3 to 12, base iron electroplated with an Fe electroplating solution containing citric acid as a complexing agent to have an iron adhesion of 0.42 to 2.99 g / m ² was annealed at the dew point of -15 ° C and +5 ° C in an annealing furnace and molten zinc Plating was performed. When iron was electroplated in the Fe electroplating solution containing citric acid as a complexing agent, the concentration of oxygen in the Fe plating layer was as low as less than 5% by weight. Although the dew point of the annealing furnace was adjusted to -15℃ and +5, the level of non-plating tended to gradually improve as the amount of Fe electroplating increased, but fine non-plating occurred on most surfaces, and plating adhesion this was bad. The Fe electroplating amount of Comparative Example 11 was 1.99 g/m ² , and the GDS profile of the hot-dip galvanized base iron was annealed by adjusting the dew point in the annealing furnace to +5 ° C. The GDS profile is shown in FIG. 4 (b). Although the concentration of Mn was concentrated at a position of about 0.2 μm in the depth direction directly under the Fe plating layer, Si was not generated with a high local maximum due to internal oxidation. The concentration difference between the maximum and minimum points of Mn and Si was 69% and 45%, respectively.

In Comparative Examples 13 to 17, cold-rolled steel sheets electroplated with Fe with an Fe electroplating solution using glycine, a kind of amino acid, as a complexing agent were annealed and hot-dip galvanized while adjusting the dew point in the annealing furnace to -18 ° C. When the Fe electroplating amount was 1.18 g/m ² or less, fine linear defects were confirmed, and plating adhesion was poor. On the other hand, when the Fe electroplating amount was 1.99 to 2.99 g/m ² , the plating appearance was good, but the adhesion was poor. Although the surface diffusion of Mn and Si was effectively suppressed during the heating process in the annealing furnace due to the high oxygen content in the Fe plating layer, the partial pressure of oxygen in the annealing furnace was insufficient to penetrate into the steel, and the surface oxidation was at a level that could aggravate the surface oxidation. It is judged that the dew point is very low or makes the hot-dip galvanizing quality worse than when it is sufficiently high. The Fe electroplating weight of Comparative Example 16 was 1.99 g/m ² , and the GDS profile of the steel sheet annealed at the dew point of -18° C. in the annealing furnace was shown in FIG. 4 (c). Although internal oxidation was effectively formed at the interface between the Fe plating layer and the base iron by the oxygen of the Fe plating layer, the oxidizing atmosphere on the surface resulted in a concentration difference of 53.7% and 45.1%, respectively, between the maximum and minimum points of Mn and Si according to the present invention. conditions were not met, and as a result, plating properties deteriorated.

In Comparative Example 18 and Inventive Examples 1 to 4, Fe electroplating was performed under the same conditions as Comparative Examples 13 to 17, the dew point in the annealing furnace was raised to -15 ° C, and then hot-dip galvanizing was performed after annealing. However, Comparative Example 18 was a case where the amount of Fe coating was small, and as a result, the difference in converted concentration between the maximum and minimum points of Mn and Si was not sufficient, and therefore, plating performance and coating adhesion were not sufficient. In addition, Fe electroplating was performed under the same conditions in Comparative Example 19 and Inventive Examples 5 to 8, and the dew point in the annealing furnace was further raised to +5 ° C., and annealing and hot-dip plating were performed. When the Fe electroplating amount was as low as 0.40 g/m ² , the surface condition was improved compared to the case where only internal oxidation was performed, but plating adhesion was poor. However, when the Fe electroplating amount is 0.82 g/m ² or more, the internal oxidation effect is increased when the internal oxidation is performed at the dew point of -15℃ and +5℃, so that a hot-dip galvanized steel sheet having a beautiful surface and excellent plating adhesion can be obtained. could The GDS concentration profile of the base iron for hot-dip galvanizing electroplated in the Fe electroplating solution using glycine as a complexing agent of Inventive Example 7 to an adhesion amount of 1.99 g/m ² and annealed while maintaining the dew point in the annealing furnace at +5 ° C It is shown in (d) of FIG. Compared to the case where only internal oxidation is performed, the oxygen concentration in the Fe plating layer is low, or the dew point in the annealing furnace is low, an Fe plating layer having a high oxygen concentration is formed and internal oxidation is performed. Since Mn and Si hardly diffuse to the surface, the hot-dip galvanizing quality could be dramatically improved.

In Inventive Examples 9 to 16, Fe electroplating was performed at a high current density of 70 A/dm ² in a Fe electroplating solution containing glycine, and annealing was performed while maintaining the dew point in the annealing furnace at -15 ° C and +5 ° C, followed by melting Zinc plating was performed. During Fe electroplating, the concentration of oxygen in the Fe plating layer was greatly increased compared to the case of electroplating at 20A/dm ² , the surface of the hot-dip galvanized steel sheet was beautiful, and the plating adhesion was good. The internal oxidation of Mn and Si was also well formed in the GDS concentration profile of the ferrous metal. When the Fe plating layer having an oxygen concentration higher than 5% by weight is electroplated at 0.5 g/m ² or more and the internal oxidation is performed by raising the dew point in the annealing furnace to -15 ℃ or more, the internal oxidation effect by Fe electroplating and the annealing furnace As the internal oxidation effect increases due to the increase in the dew point, the quality of hot-dip galvanizing of high-strength steel sheet can be dramatically improved.

As described above, it was confirmed that in the case of the inventive examples satisfying all the conditions of the present invention, excellent plating properties and plating adhesion were exhibited. Thus, the advantageous effects of the present invention could be confirmed.

Claims (13)

It includes a base iron and an Fe plating layer formed on the base iron,
The GDS profile of the Mn component observed from the surface of the Fe plating layer in the depth direction sequentially includes a maximum point and a minimum point,
The difference between the value obtained by dividing the Mn concentration at the maximum point by the Mn concentration in the parent material and the value obtained by dividing the Mn concentration at the minimum point by the Mn concentration in the parent material (concentration difference in terms of Mn) is 80% or more,
The difference between the Si concentration at the maximum point divided by the Si concentration of the base material and the value obtained by dividing the Si concentration at the minimum point by the Si concentration of the base material (concentration difference in terms of Si) is 50% or more,
The steel sheet containing Mn: 1.0 ~ 8.0%, Si: 0.3 ~ 3.0% in weight%.
However, if a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is the point where the minimum point appears.
delete The steel sheet according to claim 1, wherein a difference in the converted concentration of Mn is 90% or more and a difference in the converted concentration of Si is 60% or more.
The steel sheet according to claim 1, wherein the depth at which the maximum point is formed is 0.05 to 1.0 μm.
delete The method according to any one of claims 1, 3, and 4, wherein the base iron is, by weight%, Mn: 1.0 to 8.0%, Si: 0.3 to 3.0% C: 0.05 to 0.3%, Al: 0.005 to 3.0 %, P: 0.04% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), balance Fe and unavoidable impurities.
A hot-dip galvanized steel sheet comprising the steel sheet for plating according to any one of claims 1, 3 and 4 and a hot-dip galvanized layer formed on the steel sheet for plating.
preparing a holding iron;
Iron ions including ferrous ions and ferric ions, a complexing agent, and unavoidable impurities are included, and the ferric ions are 5 to 60% by weight based on the total iron ions. forming an Fe plating layer containing 5 to 50% by weight of oxygen by electroplating; and
Annealing the base iron on which the Fe plating layer is formed by maintaining it at 600 to 950 ° C. for 5 to 120 seconds in an annealing furnace with a 1 to 70% H ₂ -remaining N ₂ gas atmosphere controlled at a dew point temperature of -15 to +30 ° C.
including,
The deposition amount of the Fe plating layer is 0.5 to 3 g / m ² ,
The complexing agent is at least one selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid and glycylglycine,
The total concentration of iron ions is 1 to 80 g per 1 L of the electroplating solution,
The electroplating is performed under conditions of a solution temperature of 80° C. or less and a current density of 3 to 120 A/dm ² ,
The method of manufacturing a steel sheet for plating, wherein the base iron contains Mn: 1.0 to 8.0%, Si: 0.3 to 3.0% by weight%.
delete delete delete delete preparing a holding iron;
Iron ions including ferrous ions and ferric ions, a complexing agent, and unavoidable impurities are included, and the ferric ions are 5 to 60% by weight based on total iron ions. forming an Fe plating layer containing 5 to 50% by weight of oxygen by electroplating;
The base iron on which the Fe plating layer is formed is annealed by maintaining it at 600 to 950 ℃ for 5 to 120 seconds in an annealing furnace with a 1 to 70% H ₂ -remaining N ₂ gas atmosphere controlled at a dew point temperature of -15 to +30 ℃, and then plated. obtaining a steel sheet for use; and
Immersing the steel sheet for plating in a galvanizing bath
including,
The deposition amount of the Fe plating layer is 0.5 to 3 g/m ² ,
The complexing agent is at least one selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid and glycylglycine,
The total concentration of iron ions is 1 to 80 g per 1 L of the electroplating solution,
The electroplating is performed under conditions of a solution temperature of 80° C. or less and a current density of 3 to 120 A/dm ² ,
The method of manufacturing a hot-dip galvanized steel sheet containing Mn: 1.0 ~ 8.0%, Si: 0.3 ~ 3.0% in weight% of the base iron.

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